School of Medical Sciences, University of New South Wales, Sydney, NSW, Australia.
Mod Pathol. 2011 Mar;24(3):396-411. doi: 10.1038/modpathol.2010.212. Epub 2010 Nov 19.
Epigenetic silencing of cancer-related genes by promoter methylation is a frequent event in sporadic colorectal cancer. The CpG island methylator phenotype (CIMP+), in which discrete genes throughout the genome are simultaneously methylated, and long-range epigenetic silencing, whereby multiple genes within contiguous chromosomal regions are methylated, have been described in subsets of colorectal cancer. We previously reported the concurrent methylation of the mismatch repair gene MLH1 with a cluster of flanking genes in chromosome region 3p22 in sporadic colorectal carcinoma exhibiting microsatellite instability and the BRAF V600E mutation. Herein, we aimed to determine whether methylation of MLH1 and neighbouring 3p22 genes, singly or concomitantly, correlate with the germline c.-93G>A SNP within the MLH1 promoter, CIMP+ and other clinicopathological and molecular features of the tumours. By studying a cohort of 946 sporadic colorectal cancer cases, we show a strong association between concordant methylation of ≥ 3 of five 3p22 genes with CIMP+ and the BRAF V600E mutation (P<0.001). These associations were independent of microsatellite instability, as concomitant methylation of 3p22 genes other than MLH1 was found in microsatellite stable cancers. These findings show that long-range epigenetic silencing across 3p22 occurs in the context of CIMP+ and the BRAF V600E mutation, and only gives rise to microsatellite instability when this process encompasses MLH1. Furthermore, the strong relationship between long-range epigenetic silencing of 3p22 and CIMP+ provides further evidence that these two purportedly distinct epigenetic phenotypes represent a single entity with a common aetiology. Low-level methylation of MLH1 and flanking 3p22 genes, as well as the BRAF V600E mutation, were detected in the apparently normal colonic mucosa of a small number of cases whose tumours showed a similar molecular profile, suggesting that these concurring genetic and epigenetic events can occur as a field defect in neoplastic development.
抑癌基因启动子甲基化导致的表观遗传学沉默是散发性结直肠癌的常见事件。CpG 岛甲基化表型（CIMP+）是指基因组中离散基因同时被甲基化，以及长距离的表观遗传学沉默，即多个基因在连续的染色体区域被甲基化，这些现象在结直肠癌的亚群中已有描述。我们之前报道过在表现出微卫星不稳定和 BRAF V600E 突变的散发性结直肠癌中，错配修复基因 MLH1 与染色体 3p22 区域中侧翼基因簇的同时甲基化。在此，我们旨在确定 MLH1 和相邻 3p22 基因的单独或同时甲基化是否与 MLH1 启动子内的种系 c.-93G>A SNP、CIMP+以及肿瘤的其他临床病理和分子特征相关。通过研究 946 例散发性结直肠癌病例的队列，我们发现 3p22 中 5 个基因中≥3 个基因的一致性甲基化与 CIMP+和 BRAF V600E 突变之间存在强烈关联（P<0.001）。这些关联独立于微卫星不稳定性，因为在微卫星稳定的癌症中发现了除 MLH1 以外的 3p22 基因的同时甲基化。这些发现表明，3p22 上的长距离表观遗传学沉默发生在 CIMP+和 BRAF V600E 突变的背景下，只有当这个过程包含 MLH1 时才会导致微卫星不稳定。此外，3p22 长距离表观遗传学沉默与 CIMP+之间的强烈关系进一步证明了这两种据称不同的表观遗传表型代表具有共同病因的单一实体。在肿瘤表现出相似分子特征的少数病例的正常结肠黏膜中检测到 MLH1 和相邻 3p22 基因的低水平甲基化以及 BRAF V600E 突变，这表明这些同时发生的遗传和表观遗传事件可以作为肿瘤发生过程中的一个场缺陷。
